Hi.
Sounds like there's a lot going on around you! With your mom getting engaged, there are bound to be many changes happening. It's typical to feel all sorts of emotions: anger, sadness, relief or happiness. How do YOU feel? If you ARE happy, go ahead and be happy. But definitely don't fake it.
Have you talked to your mom about how you feel?
This is a big thing for her, but she probably wants to know how you feel about it, too. Do you like the guy she's dating? Why or why not? Where does your friend stand? Could your relationship with your friend change if your parents marry each other? Would it be for the better?
There will be a period of adjustment before, during and after your parents marry, if they do. It's fine if your feelings change during this whole time--it's understandable that they would!